The IT-TOFMS has the characteristics of both the ion trap (IT), which is capable of a multi-stage mass spectrometric analysis (an MSn analysis), and the time-of-flight mass spectrometer (TOFMS), which is capable of performing a mass analysis with high mass-resolving power and high mass accuracy. It has been effectively applied in various fields, particularly in the compositional or structural analysis of high-molecular compounds (e.g. proteins, sugar chains or the like).
There are many types of ion traps, such as the three-dimensional quadrupole type or linear type. In the following description, a three-dimensional quadrupole ion trap having a ring electrode and a pair of end-cap electrodes is taken as one example. In this ion trap, a radio-frequency voltage is applied to the ring electrode in order to capture ions within a space surrounded by the ring electrodes and the end-cap electrodes. To apply the ion-capturing radio-frequency voltage, LC resonance circuits have been conventionally used. In recent years, a new type of device called “digital ion trap” has been developed, which uses a rectangular-wave voltage as the radio-frequency voltage (for example, refer to Patent Documents 1-3 as well as Non-Patent Document 1). As described in Patent Document 1, a digital ion trap includes a drive circuit in which a high direct-current (DC) voltage generated by a DC power source is switched by a high-speed semiconductor switch to generate a rectangular-wave voltage. In principle, this circuit can instantly initiate or halt the application of the voltage with a desired timing (at dramatically higher speeds than the LC resonance circuit).
In the IT-TOFMS, if all the ions to be analyzed are accelerated with the same amount of energy, the ions will fly at different speeds due to the difference in their mass-to-charge ratio and be appropriately separated before arriving at the detector. Therefore, if the ions vary in the amount of energy immediately before the accelerating energy is given, the energy variation will emerge as a difference in the flight speed, which leads to an erroneous result. In an MSn analysis, this problem is avoided as follows: After a group of ions originating from a sample have been captured in the ion trap, the process of selecting an ion having a specific mass-to-charge ratio and performing collision induced dissociation using the selected ion as the precursor ion is repeated so as to leave a desired kind of ions within the ion trap. Then, the ions maintained in this manner are cooled by collision with a cooling gas (e.g. argon) introduced in the ion trap. As a result of this cooling process, the amount of energy possessed by each ion gradually is attenuated and the ions gather around the center of the ion trap. Subsequently, a direct-current voltage is applied to the end-cap electrodes to create a strong direct-current electric field within the ion trap. This electric field gives an amount of accelerating energy to each ion, whereby the ions are collectively ejected from the ion trap into the TOFMS.
As just described, the ions undergo the cooling process before being ejected from the ion trap. Even during the cooling process, the ions continue oscillating due to the effect of the ion-capturing electric field and become spatially spread to some extent (i.e. they have a spatial distribution). Since the accelerating electric field created by the voltage applied between the two end-cap electrodes has a potential gradient, the amount of potential energy that each ion receives at the moment of ejection depends on the position of the ion. Accordingly, the ions ejected from the ion trap will have a certain amount of energy width.
In the case of the linear type TOFMS, in which the ions are made to fly straight, the aforementioned energy width of the ions having the same mass-to-charge ratio results in a difference in their flight speed and constitutes a factor that lowers the mass-resolving power. By contrast, in the reflectron type TOFMS, the reflectron has the effect of correcting the difference in the potential energy. Though no detailed description will be made in this specification, a well-known type of reflectron, called the “dual-stage reflectron”, can correct the second-order aberration of the energy. Even if the amounts of energy of the ions ejected from the ion trap vary within a certain range, the reflectron can correct this variation and temporally focus the ions into an adequately narrow range of time of flight to avoid the decrease in the mass-resolving power.
However, there is another factor that deteriorates the mass-resolving power of the IT-TOFMS; that is, the turn-around time. Suppose there are two ions whose initial velocities are equal in absolute value but have opposite directions immediately before being ejected from the ion trap, with one ion having a velocity component directed toward the TOFMS and the other ion having a velocity component directed away from the TOFMS. When an accelerating electric field for ejecting ions is created, the former ion is immediately accelerated along the downward potential gradient of the accelerating electric field, to be directly sent toward the TOFMS. On the other hand, the latter ion (i.e. the ion having a velocity component directed away from the TOFMS) existing near the center of the ion trap is initially decelerated along the upward potential gradient of the accelerating electric field and then turns to the opposite direction, to be accelerated toward the TOFMS. The period of time τTA that passes until this ion once more passes through the center of the ion trap at the initial velocity is called the turn-around time, which is expressed as the following equation:τTA=(2ν0m)/(zeE)  (1),where ν0 is the initial velocity of the ion in the direction away from the TOFMS, m is the mass of the ion, z is the charge number of the ion, e is the elementary charge, and E is the strength of the accelerating electric field at the moment of ejection.
Thus, an ion traveling in the direction away from the TOFMS at the moment of the ejection of the ions will return to the original position after the turn-around time τTA and then travel toward the TOFMS at the same initial velocity. The arrival of this ion at the detector will be delayed by the turn-around time τTA from that of the ion which travels toward the TOFMS from the beginning. Such a difference in the time of flight due to the turn-around time for the ions having the same mass-to-charge ratio cannot be corrected even by reflectrons. It is also impossible to distinguish between these two ions on the detector. As a result, the mass-resolving power will deteriorate.
With the TOFMS techniques available in recent years, a potential energy having a width of approximately ±10% can be corrected by using an adequately tuned reflectron. Therefor; the turn-around time, which cannot be corrected by reflectrons, is currently the most dominant limiting factor for the improvement of the mass-resolving power in the IT-TOFMS.